A brushless direct current (DC) motor is generally provided with a rotor having a permanent magnet and a stator having a plurality of phase coils in a star connection, the coils are excited by controlling an electrical current supplied to the coils, and the motor is driven by rotating the rotor relative to the stator. In order to detect a rotation position of the rotor, the brushless DC motor is generally provided with a sensor such as a Hall element or an optical encoder, and by switching the current supplied to each phase coil in accordance with the position detected by the sensor, gives an appropriate torque to the rotor.
In order realize smaller motors, a sensorless motor that detects the rotation position of the rotor without using a sensor, such as the Hall element or the like, has been proposed (for example, Patent Document 1). By measuring electrical potential at the midpoint of the wiring of the motor, for example, the sensorless motor detects induced voltage generated in the coils, and obtains position information. Since this sensorless motor obtains the position information from the induced voltage generated while the rotor is rotating, there is a problem in that it is not possible to know the rotation position when stopped. When the motor is started while not correctly knowing the rotation position, there are cases in which the rotor rotates in a direction opposite to a desired direction of rotation. However, for example in cases in which a spindle motor of a hard disk is rotated, or the like, since it is desirable to curtail, to the minimum, rotation in a reverse direction, it is necessary to appropriately comprehend the rotation position of the motor when the motor is started. For a sensorless motor, technology for detecting the position of the motor when the motor is started is disclosed in, for example, Patent Documents 1, 2 and 3.
Patent Document 1: Japanese Patent Application, Laid Open No. H3-207250
Patent Document 2: Japanese Patent Application, Laid Open No. H6-113585
Patent Document 3: Japanese Patent Application, Laid Open No. H11-122977
Generally, in a brushless DC motor, the rotation position is detected at 60 degree intervals, and current control is carried out in accordance with the detected rotation position; however, in methods described in the abovementioned Patent Documents 1, 2, and 3, the rotation angle detected at a starting time, and the rotation angle when control is performed during driving are out of alignment by 30 degrees. As a result, it is necessary to carry out control for driving the motor after correcting the rotation angle detected at starting time.